A B cell leukemia (L2C) of inbred strain 2 guinea pigs and a new myelogenous leukemia (G-13) of inbred strain 13 guinea pigs are being investigated as models of human leukemia. The chemical nature of the tumor specific transplantation antigen (TSTA) of the L2C leukemia is being investigated by immunization protection tests in syngeneic animals. The TSTA of the L2C leukemia is unusual in that it is of low molecular weight and is extremely resistant to heating. The G-13 leukemia is being examined for the presence of a TSTA and cell surface markers. As in the human, the myeloblasts of this leukemia are Ia positive. The immune functions of these cells are also being investigated. Patients with systemic lupus erythematosus (SLE) have a defect in the development of Con A induced suppressor cells. Moreover, the sera from lupus patients (more particularly the IgM fraction) can induce suppressor cell defects in populations of normal lymphocytes. T gamma and T non-gamma cells mediate different types of suppression; certain lupus sera are specifically cytotoxic for T gamma cells. Finally T cells able to proliferate in response to autologous non-T cells act as precursors of Con A induced suppressor cells.